1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to the so called air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the mixture supplied to the engine is controlled around a stoichometric value (14.7:1 for example) by a feedback control according to the composition of the exhaust gas and operating conditions of the engine. This is because an optimum operation of the three-way catalytic converter is enabled at the stoichometric air-fuel ratio. The air intake side secondary air supply system for the feedback control is an example of this type of feedback control system and which is constructed such that an air intake side secondary air supply passage leading to a downstream side of the throttle valve is provided. The air fuel ratio control is performed by varying the amount of the secondary air flowing through the air intake side secondary air supply passage.
As an example of the air intake side secondary air supply system, a system performing a pneumatic type PI (proportional and integral) control operation is described in Japanese patent application No. 57-2127548 assigned to the same assignee of the present application. In this system, there are provided a pair of air intake side secondary air supply passages, both communicated to the downstream side of the throttle valve. The air-fuel ratio is detected in terms of a exhaust gas oxygen concentration and an air-fuel ratio signal is produced. The communication through one of the air intake side secondary air supply passages is controlled by means of an open/close valve operated in accordance with the air-fuel ratio signal. The other one of the air intake side secondary air supply passage is provided with an air control valve whose opening degree is controlled by the magnitude of the pressure applied to a pressure chamber thereof. The pressure chamber of the air control valve is supplied with one of first and second control pressures for respectively opening and closing the air control valve so that the area of the other one of the air intake side secondary air supply passage is gradually increased or decreased.
In this type of air intake side secondary air supply systems, practically, an additional air control valve is used in the first one of the air intake side secondary air supply passage so as to control the amount of the secondary air flowing therethrough in time periods when the open/close valve is opened. In addition, it is necessary to provide a three-way solenoid valve in order to introduce one of the first and second control pressures into the pressure chamber of the air control valve disposed in the second one of the air intake side secondary air supply passages. Therefore, the problem of this type of pneumatic system was that a number of parts are required and the construction becomes complicated.
Thus, it is desired to provide a relatively low cost system of pneumatic type even through this type of system does not cost higher than conventional air intake side secondary air supply systems of electrically opperated PI operation in which the control operations are performed by pulse motors.
Further, it is desirable to determine a range of the air-fuel ratio control according to operating conditions of the engine for improving the driveability of the engine and the efficiency of the purification of the exhaust gas.
An object of the present invention is therefore to provide an air intake side secondary air supply system of the PI type pneumatic control operation, which has a relatively simple construction and costs less, while maintaining enough of an air-fuel ratio control range corresponding to the operating conditions of the engine without causing the deterioration of the driveability of the engine.
According to the present invention, the air intake side secondary air supply system includes a detection means for detecting the air-fuel ratio in terms of an oxygen concentration in the exhaust gas, and producing an air-fuel ratio signal, first and second secondary air supply passages, both communicated to the downstream side of the throttle valve. The first secondary air supply passage is provided with an air control valve for controlling the amount of the air flow according to the magnitude of pressure supplied to a pressure chamber thereof and the second secondary air supply passage is provided with a first open/close valve operated by the air-fuel ratio signal. A first air supply delay device is provided in the second secondary air supply passage, on the upstream side of the first open/close valve and a pressure supply passage is provided for the communication between the pressure chamber of the air control valve and a part of the second secondary air supply passage between the first open/close valve and the first air supply delay device. The system is further provided with a second air supply delay device in the pressure supply passage, and a pressure control means for supplying an air pressure for reducing the opening degree of the air control valve, into the pressure supply passage between the second air supply delay device and the pressure chamber of the air control valve, during specific operating conditions of the engine.
Further scope and applicability of the present invention will become apparent from the detailed description given thereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illusration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the air from this detailed description.